An electronic still image pick-up device called the so-called digital still camera that employs an image sensor as a photoelectric converter has been hitherto widely used.
In such a digital still camera, as well as a camera with a usual silver salt film, a flash function or a low speed shutter function or the like are likewise provided in order to supplement for the shortage of the quantity of light under an environment of low illuminance such as at night or a dark place. The flash function is a function for emitting a flash of strong light to apply light to a subject by using a discharge tube such as a xenon lamp. Further, the low speed shutter function indicates a function for performing charge storage in the CCD for a long time.
In the digital still camera, when an image detected by the CCD is displayed on, for instance, a liquid crystal panel used as a finder, the image of the subject before it is photographed cannot be displayed on the finder under the environment of low illuminance. That is, under the environment of low illuminance, even when a still image can be photographed, a user cannot carry out a framing process to display a picked-up image on the finder so that the user may recognize the position of the subject or a composition before the user photographs the subject.
In order to solve the above-described problem, a digital still camera having an infrared photographing function that a subject is irradiated with infrared rays and a framing process is performed while an infrared cut filter is retracted from an optical path has been recently proposed.
FIG. 1 shows the wavelength sensitivity characteristics A of a CCD and the wavelength passing characteristics B of an infrared cut filter. As shown in FIG. 1, the CCD has an adequate sensitivity not only for visible lights, but also for infrared rays having the wavelength of about 700 nm or more. On the contrary, the infrared cut filter does not pass the infrared rays having the wavelength of 700 nm or more. In an ordinary photographing, since an exposure under infrared rays is prohibited, the infrared cut filter is inserted into the optical path of light for photographing an image. On the other hand, in an infrared photographing, the infrared cut filter is retracted outside the optical path, further, a subject is irradiated with infrared rays from infrared light emitting means provided in a camera main body and reflected lights thereof are photographed. Accordingly, in the digital still camera having such an infrared photographing function, the infrared photographing is carried out in framing process. Upon still image photographing, an emission of infrared rays is stopped and a visible light photographing is carried out by using a flash function or a low speed shutter function instead thereof. Thus, even under the environment of low illuminance, the position of the subject or a composition can be recognized before the subject is photographed and the subject can be put in a frame as desired by a user.
The digital still camera ordinarily has an automatic focusing function that a focalization is automatically made. The automatic focusing system of the digital still camera includes various systems. As one of them, there is a self-focus detecting system using image characteristics that a focused image has more high frequency components than those of a hardly focused image. The self-focus detecting system is a system that the high frequency components for an actually picked-up image are detected while the position of a focus lens is moved and a focal point is set to the position of the lens where the high frequency components are most located.
However, a chromatic aberration is present in the lens, and a focal distance is different between the infrared radiation and the visible radiation. Therefore, for instance, even when the image of the same subject is picked up from the same position, the focusing position of the focus lens is different between in the infrared photographing and in the visible light photographing. Accordingly, in case the infrared photographing is carried out upon framing process and the visible light photographing using the flash or the like is carried out upon photographing a still image, when the automatic focusing process is carried out in accordance with the self-focus detecting system based on a picked up image under the infrared radiation, a blooming and dim still image is photographed.
Further, even when the automatic focusing process is carried out only by natural lights with infrared rays cut, an obtained image has a very low contrast under a state of low illuminance. Therefore, it is difficult, in the first place, to detect the focusing position by using the self-focus detecting system.
As mentioned above, when the infrared photographing is carried out in the framing process and the visible light photographing by using the flash or the like is carried out in the photographing a still image, it is extremely difficult to accurately perform the automatic focusing process for detecting the focusing position of the still image.